15
THE AMPLIFIER STAGE:
THEORY AND PRACTICE
The MIC AMP F355's gain is effected by a so
called instrumentation amplifier. It is represen-
ted by an IC specially designed for this task,
distinguishing itself by very low noise figures
and a high gain/bandwidth product.
When gain is set to +60 dB (x1000), the
amount of noise added is only 1-2 dB more
than the (theoretically) achievable minimum.
The gain/bandwidth product (GBW) and the
slew rate are basically responsible for the
sonic quality of an amplifier. The higher they
are, the more transparent the sound becomes.
Slew rate is 10V/s, while the GBW is
calculated from the frequency response at a
certain gain ratio.
The MIC AMP F355's internal linear frequency
response at +60 dB gain exceeds 200 kHz.
This results in a theoretical GBW as high as
200 MHz (200.000 Hz x 1000).
As an option MIC-AMP F355 may be equipped
with a Class-A input amplifier. It is the actual
version of circuit designs which can be found in
classic applications.
Compared to the version with integrated instru-
mentation amplifier there are small disadvan-
tages concerning THD when dealing with high
level input signals. As the relevant operation
range of a microphone preamplifier is a gain
make-up of +10 … +50 dB, here the Class-A
version shows lower THD.
Also, with higher level signals the noise floor is
significantly lower.
… and how does it sound ???
Well, since the beginning we at LAKE PEOPLE
spend lots of efforts to create no sounding
instruments !!
The author´s opinion is, that the standard
version of MIC-AMP F355 is best suited for all
standard applications.
Whereas the Class-A version is predestinated
for "Special" Recording.
THE SIGNAL PATH AND
THE CONTROLS
THE INPUT
The balanced input is situated on the rear
panel, fitted with XLR type socket. It is de-
noted "INPUT 1" and "INPUT 2" respectively.
The input accepts balanced as well as
unbalanced audio signals. XLR wiring
corresponds to international standards with
- Pin 1 = ground,
- Pin 2 = + (in phase) and
- Pin 3 = - (out of phase)
If an unbalanced source is connected, one of
the phases (preferably the negative one)
should be connected to signal ground.
PHANTOM POWERING
The use of professional condenser micro-
phones requires in general a polarisation
voltage or operating voltage, the so called
phantom power.
The MIC AMP F355 offers a stabilized 48V
phantom voltage, which is activated by the
"PHANTOM"-button on the front panel.
This voltage is applied to the input pins 2 and 3
via matched 6,8 kOhms resistors.
Active phantom powering is indicated by the
red "48V"-LED.
Normal microphone preamplifiers will produce
low-frequency voltage swing at their outputs
when switching the phantom power on or off.
This may damage subsequent equipment !
Therefore MIC-AMP F355 provides the "AU-
TO-MUTE"-feature which offers a 4-seconds
output mute whilst switching pantom on or off.
NOTE:
If unbalanced equipment such as
synthesizers etc. are connected to the
MIC AMP F355's input, the 48V
phantom voltage might cause damage
if the connection is not made properly.
This problem is faced with all phantom
powered inputs and can be avoided by
using a transformer coupled D.I. box or
by strictly keeping the unbalanced
wiring apart from pin 1 (ground).
